This invention relates to a quantizing device for use in a differential encoder in quantizing a device input signal into a quantization output signal.
In the manner known in the art, the differential encoder is preferred on encoding a picture signal representative of a succession of picture elements. Each picture element is often abbreviated to a "pel". Such an input signal is supplied to the quantizing device as the above-mentioned device input signal and is variable in a predetermined dynamic range. More particularly, the device input signal represents a signal value which is variable from one pel to another or from a current sampling instant to a next subsequent sampling instant between an upper limit and a lower limit which are different or are spaced apart by the dynamic range.
It is known in the art that the quantizing device comprises a quantizing arrangement supplied with the device input signal and a local input signal which will presently be described. The quantizing arrangement is for quantizing each difference between the device input signal and the local input signal into the quantization output signal with a quantization error which is alternatively called quantization noise. An adder is used in calculating a sum of the quantization output signal and the local input signal to produce a local decoded signal representative of the sum. Responsive to the local decoded signal, a predictor produces a prediction signal predictive of the device input signal. By way of example, a one-pel delay is used as the predictor in predicting for a signal value representative of a current picture element another signal value which the device input signal will have in representing a next subsequent picture element. The prediction signal is supplied to the quantizing arrangement as the local input signal.
It is usual to use a subtracter responsive to the device input signal and the local input signal, namely, the prediction signal, in producing a differential signal representative of the difference. The quantization output signal represents from time to time a predetermined one of quantization levels which are related to various values of the difference in accordance with a predetermined quantization characteristic. In a counterpart decoder, a decoded signal is obtained in the manner in which the local decoded signal is derived in the quantizing device. From the decoded signal, the device input signal is reproduced as a reproduced signal.
For such a quantizing device, it is possible to represent each signal value by a preselected number of binary bits. The preselected number will hereafter be denoted in general by N. In this event, the upper limit is represented by (2.sup.N-1 -1) and the lower limit, by (-2.sup.N-1). The dynamic range is between (2.sup.N-1 -1) and (-2.sup.N-1), both inclusive, namely, is 2.sup.N wide. The differential signal is represented by (N+1) or more binary bits. A great number has been used as the predetermined number of the quantization levels in order to prevent the reproduced signal from degrading due to quantization noise at each flat portion of the device input signal and due to overload of the quantizing device.
With a view to reducing the predetermined number to a smaller number, an improved quantizing device is described in an article contributed by Gert Bostelmann to a German technical periodical "NTZ," Volume 27 (1974), No. 3, pages 115 to 117, under the title of "A Simple High Quality DPCM-Codec for Video Telephone Using 8 Mbit per Second." According to the Bostelmann article, an input signal is preliminarily subject to amplitude limitation before used as the device input signal. The amplitude limitation is such that the local decoded signal may never exceed the dynamic range even if the quantization error has its greatest absolute value. More specifically, the amplitude limitation is for making the device input signal have a limited range between the upper limit less the greatest absolute value and the lower limit plus the greatest absolute value. It is possible with the amplitude limitation to make the quantizing arrangement quantize the differential signal of N binary bits. In addition, calculation is carried out with respect to a modulus of the preselected number in the adder for deriving the local decoded signal and also in the subtracter for deriving the differential signal.
The quantizing device of Bostelmann is excellent in that a smaller number is sufficient as the predetermined number of the quantization levels. The local decoded signal is, however, subjected to the amplitude limitation. The decoded signal is accordingly subjected to the amplitude limitation. This degrades the reproduced signal.
If the greatest absolute value of the quantization error is thirty-two in the Bostelmann quantizing device, the amplitude limitation results in a loss of thirty-two from each of the upper and the lower limits. When the dynamic range is 256 wide, as much as 12.5.degree./o is lost in each of positive and negative parts of the decoded signal. The decoded signal is therefore subjected to a serious degradation.